Novel seed germination in response to California’s historic drought may deplete soil seed banks

Background/Question/Methods

We observed novel seed germination, without a fire cue, in an obligate seeding species of chaparral (Ceanothus megacarpus) during historic drought in California (2014). We hypothesized that premature germination was promoted by drought-induced canopy dieback among adults that allowed high radiation loads to reach normally shaded soils, cracking seed coats, facilitating water uptake. We hypothesized that because premature seed germination was late in the season (April, when December is typical) roots would not have time to penetrate deep moisture resources before the onset of summer drought, leading to excessive mortality and eventual depletion of soil seed banks. We tested these hypotheses by setting up 67 permanent quadrats (1 m²), at four study sites, varying between a mean of 7 to 10 seedlings per m², and monitored a total of 576 seedlings, recording mortality every two weeks, from May 2014 to January 2015. We used an infrared thermometer to measure surface soil temperatures below the dead canopy of adults, a dewpoint hygrometer to estimate soil water potentials at 10 cm and 20 cm depths (n = 67), and excavated roots of dead and alive seedlings (n = 12) in late May to estimate maximum rooting depth. 

Results/Conclusions

Surface soil temperatures at all four study sites approached 70 C. Seed germination trials in the laboratory resulted in > 98% germination at 70 C for 1 h, much higher than controls (~20%). Unpaired Student’s t-test (n = 67) showed the number of seedlings per unit area was significantly higher on trails than off trails where canopy gaps were more prominent. Both findings were consistent with our initial hypothesis. Seedling survival corresponded to maximum rooting depth. Dead seedlings had roots < 20 cm deep whereas surviving seedlings had roots > 20 cm deep, in correspondence with mean soil water potentials of -30 MPa at 10 cm depth, a value more negative than critical levels for hydraulic dysfunction (-11 MPa for C. megacarpus seedlings). By January 2015, total survival rate of the 576 monitored seedlings was only 3.8%, a value significantly lower than the survival rate reported for C. megacarpus seedlings (literature values range between 55-63%). We conclude that novel seed germination due to historic drought in California will not compensate for adult mortality in C. megacarpus but will exacerbate mortality through depletion of the seed bank, causing future shifts in plant community structure.